Objective: Primary objective: To evaluate whole body distribution of 89Zr-pembrolizumab in patients with locally advanced or metastatic melanoma or NSCLC. Secondary objectives: i) To evaluate pharmacokinetics of 89Zr-pembrolizumab; ii) To assess the heterogeneity of 89Zr-pembrolizumab tumor uptake; iii) To describe safety of 89Zr-pembrolizumab; iv) To correlate the tumor response of pembrolizumab, as measured by objective response rate (ORR) according to standard RECIST v1.1 as assessed by the investigator, with specific tumor tracer uptake.